Bias circuit

ABSTRACT

A bias circuit includes: a bias supply terminal  800 ; a parallel capacitor  3  connected at one end thereof to the bias supply terminal  800  and grounded at the other end thereof; and a parallel circuit  3 L connected in parallel with the parallel capacitor  3  and connected at one end thereof to the bias supply terminal  800 . Let 2≦N. The parallel circuit  3 L includes: a direct-current power supply connection terminal  600 ; N parallel inductors  2   1  to  2   N  connected in series with each other between the bias supply terminal  800  and the direct-current power supply connection terminal  600 ; and N−1 series resonators  9   1  to  9   N−1 . Each series resonator  9   1  to  9   N−1  includes: a resonant capacitor  7   1  to  7   N−1  connected at one end thereof to a connecting point between adjacent parallel inductors; and a resonant inductor  8   1  to  8   N−1  connected at one end thereof to the other end of the resonant capacitor  7   1  to  7   N−1  and grounded at the other end thereof.

TECHNICAL FIELD

The present invention relates to a bias circuit used in an amplifier or the like.

BACKGROUND ART

An active device such as transistors requires a direct-current (DC) power for operation. A bias circuit is used to supply the DC power to the active device while isolating transmission paths of an alternating-current (AC) signal from a DC power supply.

For example, in an amplifier, an amplification device outputs the amplified AC signal at an output terminal. The main energy of the amplified signal comes from the DC power supply. That is, the amplifier converts the energy received from the DC power supply to that of the AC signal. The bias circuit serves to pass the energy from the DC power supply to the amplification device. In order not to affect the transmission of the AC signal, the bias circuit has a filtering function for preventing the AC signal from flowing into the bias circuit.

FIG. 1 shows an exemplary configuration of a conventional bias circuit 900 that supplies a DC voltage to an active device (FET) 180. A bias supply terminal 800 of the bias circuit 900 is connected to a line between an AC signal source 1 and the active device 180 (between a port A and a port B). The bias circuit 900 includes a parallel resonant circuit 78 p with a capacitor 7 having a capacitance C₁ and an inductor 8 having an inductance L₁ connected in parallel with each other. One end of the parallel resonant circuit 78 p corresponds to the bias supply terminal 800 and the other end of the parallel resonant circuit 78 p corresponds to a terminal 600 where a DC circuit part 10 with a DC circuit 5, such as a choke coil, and a DC power supply 6 is connected. The DC power supply 6 generates a certain DC voltage with respect to the ground potential. A capacitor 4 having a sufficiently-large capacitance and grounded at one end thereof is connected to the terminal 600. The energy of the DC power supply 6 is supplied to the active device 180 via the terminal 600 and the bias supply terminal 800.

The resonance frequency of the parallel resonant circuit 78 p denoted by a character f is determined by the capacitance C₁ and the inductance L₁. At the resonance frequency f, the impedance of the bias circuit 900 as viewed from the bias supply terminal 800 toward the DC power supply 6 is considered infinite, so that no AC signal enters the bias circuit. In other words, the bias circuit 900 is a bias circuit that operates at the resonance frequency f. In general, the filtering function of the resonant circuit of the conventional bias circuit 900 or the like is effective only in the proximity of the resonant frequency.

However, recently, there is an increasing demand for radio frequency (RF) circuits capable of multiband operation. The amplifier is an essential component of the RF circuits. That is, the amplifier also is desired to operate in multiple bands. Several methods for enabling multiband operation of the amplifier have been proposed. One example is an amplifier capable of simultaneously amplifying signals in two inherent frequency bands (see “Dual-band Power SiMOSFET Amplifier with Two-section Impedance Transformers”, Uchida et al., 2004 IEICE General Conference, C-2-39, referred to as Non-Patent literature 1 hereinafter). The multi-band amplifier can be applied to a system capable of high-speed transmission using a plurality of frequency bands, such as spectrum aggregation. To supply energy from a DC power supply to such a multiband amplifier, the bias circuit itself has to operate in multiple bands. This means that the conventional bias circuit 900 is difficult to apply to the multiband amplifier. Thus, the multi-band amplifier disclosed in Non-Patent literature 1 has a bias circuit that comprises a parallel resonant circuit and a transmission line Q having an effect similar to that of the parallel resonant circuit connected in series with each other. This bias circuit is designed in such a manner that the resonance frequency of the parallel resonant circuit is a first frequency and the transmission line Q is a quarter-wave line that is short-circuited at the tip at a second frequency. Therefore, this bias circuit can supply energy of a DC power supply to the multiband amplifier simultaneously at two different frequency bands. However, it is difficult for the bias circuit configured in this way to supply energy of the DC power supply to the multiband amplifier simultaneously at three or more different frequency bands.

Another known bias circuit capable of multiband operation is a bias circuit disclosed in Japanese Patent Application Laid-Open No. 2006-254378. This bias circuit supplies energy of a DC power supply separately for a plurality of frequency bands by switching and cannot supply energy from a DC power supply simultaneously for a plurality of frequency bands.

SUMMARY OF THE INVENTION

In view of such circumstances, an object of the present invention is to provide a bias circuit that has high design flexibility and can supply a DC voltage and/or a direct current to an active device simultaneously for a plurality of frequency bands.

To attain the object, a bias circuit according to the present invention is configured as described below. That is, the bias circuit comprises: a bias supply terminal for supplying a DC voltage and/or a direct current to an active device; a capacitor (a parallel capacitor) connected at one end thereof to the bias supply terminal and grounded at the other end thereof; and a parallel circuit connected in parallel with the parallel capacitor and connected at one end thereof to the bias supply terminal. Let N denote a predetermined integer equal to or greater than 2, q denote each integer equal to or greater than 2 and equal to or smaller than N−1, and m denote each integer equal to or greater than 1 and equal to or smaller than N−1. The parallel circuit comprises: a DC power supply connection terminal connected to a DC circuit part including a DC power supply; N inductors (parallel inductors) connected in series with each other; and N−1 series resonators, wherein a first parallel inductor of the N parallel inductors is connected at one end thereof to the bias supply terminal, a q-th parallel inductor of the N parallel inductors is connected at one end thereof to the other end of a (q−1)-th parallel inductor of the N parallel inductors and connected at the other end thereof to one end of a (q+1)-th parallel inductor of the N parallel inductors, an N-th parallel inductor of the N parallel inductors is connected at the other end thereof to the DC power supply connection terminal, and an m-th series resonator of the N−1 series resonators includes an m-th capacitor (a resonant capacitor) and an m-th inductor (a resonant inductor) and is connected at one end thereof to a connecting point between an m-th parallel inductor of the N parallel inductors and an (m+1)-th parallel inductor of the N parallel inductors and grounded at the other end thereof.

EFFECTS OF THE INVENTION

According to the present invention, as described later with regard to specific embodiments, a series resonator in a bias circuit has an impedance of zero at the resonance frequency of the series resonator. Thus, at the frequency, a short circuit state (a short-circuited state from the viewpoint of an AC signal) occurs at a connecting point (between parallel inductors) to which the series resonator is connected. As a result, at the frequency, the bias circuit is effectively equivalent to an arrangement comprising a parallel capacitor, and one or more parallel inductors and one or more series resonators that exist between the short-circuited connecting point and a bias supply terminal. This holds true for each of the series resonators in the bias circuit. Therefore, when the reactance of each component in the bias circuit is appropriately set so that the impedance of the bias circuit as viewed from the bias supply terminal toward the bias circuit is infinite (that is, the admittance is zero) at each resonance frequency, a DC voltage and/or a direct current can be supplied to an active device simultaneously for a plurality of frequency bands without a switching operation or the like. In addition, since the impedance of the series resonator to a direct current is infinite, no direct current flows through the series resonator. Thus, there are less design constraints on the inductors included in series resonators in particular, and the bias circuit is easy to design. In addition, since many of the components of the bias circuit may be lumped elements, the bias circuit can have a smaller size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an exemplary configuration of a conventional bias circuit;

FIG. 2 is a block diagram showing a bias circuit (designed for dual-band operation) according to an embodiment of the present invention;

FIG. 3 is a diagram showing a specific example of the bias circuit designed for dual-band operation according to the embodiment in which a first frequency f₁=2.5 GHz, and a second frequency f₂=2 GHz;

FIG. 4 is a graph showing a result of simulation of pass characteristics (S21) at the first frequency f₁ and the second frequency f₂ of the bias circuit designed for dual-band operation according to the embodiment shown in FIG. 3;

FIG. 5 is a block diagram showing a bias circuit (designed for N bands) according to an embodiment of the present invention;

FIG. 6 is a diagram showing a specific example of the bias circuit designed for four bands according to the embodiment in which a first frequency f₁=2.5 GHz, a second frequency f₂=2 GHz, a third frequency f₃=1.5 GHz, and a fourth frequency f₄=1 GHz;

FIG. 7 is a graph showing a result of simulation of pass characteristics (S21) at the first to fourth frequencies of the bias circuit designed for four bands according to the embodiment shown in FIG. 6; and

FIG. 8 is a block diagram showing a modification of the bias circuit (designed for N bands) according to the embodiment of the present invention shown in FIG. 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described with reference to the drawings. Throughout the drawings, the same components are denoted by the same reference numerals, and redundant descriptions thereof will be omitted.

To help understanding the present invention, FIG. 2 shows a bias circuit 100 according to an embodiment of the present invention that can supply a DC voltage and/or a direct current from a DC power supply 6 to an active device 180 simultaneously for two different frequency bands f₁ and f₂. In this embodiment, the bias circuit 100 is used on the input terminal side of an amplifier.

A bias supply terminal 800 of the bias circuit 100 is connected to a line between an AC signal source 1 and an active device 180 (between a port A and a port B).

The active device 180 may be a transistor, a field effect transistor (FET), a metal oxide semiconductor FET (MOSFET), a thin film transistor (TFT), or the like. Although the active device 180 shown in the drawings is an n-channel junction FET, the n-channel junction FET is only an example of the active device 180, and the present invention is not limited to the n-channel junction FET.

The bias circuit 100 includes: a capacitor 3 having a capacitance C_(p), grounded at one end thereof and connected at the other end thereof to the bias supply terminal 800 (referred to as a parallel capacitor hereinafter); and a parallel circuit 3R connected in parallel with the parallel capacitor 3 and connected at one end thereof to the bias supply terminal 800. The parallel circuit 3R includes: an inductor 2 ₁ having an inductance L_(p1) and connected at one end thereof to the bias supply terminal 800 (referred to as a first parallel inductor hereinafter); an inductor 2 ₂ having an inductance L_(p2) and connected at one end thereof to the other end of the first parallel inductor 2 ₁ and connected at the other end thereof to a DC power supply connection terminal 600 (referred to as a second parallel inductor hereinafter); a first series resonator 9 ₁ connected at one end thereof to a connecting point 700 ₁ between the first parallel inductor 2 ₁ and the second parallel inductor 2 ₂; a capacitor 4 having a capacitance C and connected at one end thereof to the DC power supply connection terminal 600 and grounded at the other end thereof (referred to as a grounding capacitor hereinafter); and a DC circuit part 10 connected to the DC power supply connection terminal 600. The first series resonator 9 ₁ includes: a capacitor 7 ₁ having a capacitance C_(s1) and connected at one end thereof to the connecting point 700 ₁ (referred to as a first resonant capacitor hereinafter); and an inductor 8 ₁ having an inductance L_(s1) and connected at one end thereof to the other end of the capacitor 7 ₁ and grounded at the other end thereof (referred to as a first resonant inductor hereinafter). The DC circuit part 10 includes: a DC circuit 5, such as a choke coil, connected at one end thereof to the DC power supply connection terminal 600; and a DC power supply 6 that is connected to the other end of the DC circuit 5 and generates a certain DC voltage with respect to the ground potential. The energy from the DC power supply 6 is supplied to the active device 180 via the bias supply terminal 800.

The grounding capacitor 4 has a sufficiently-large capacitance C, and the impedance of the grounding capacitor 4 is substantially zero (a short circuit state) in any desired frequency bands.

A resonance frequency of the first series resonator 9 ₁ is referred to as a first frequency f₁. The first frequency f₁, an inductance L_(s1) and the capacitance C_(s1) are related according to the following formula (1).

$\begin{matrix} {f_{1} = \frac{1}{2\pi\sqrt{L_{s\; 1}C_{s\; 1}}}} & (1) \end{matrix}$

At the first frequency f₁, the impedance of the first series resonator 9 ₁ as viewed from the connecting point 700 ₁ toward the first series resonator 9 ₁ is zero (a short circuit state). Thus, when the bias circuit 100 is viewed from the bias supply terminal 800, the short circuit state always occurs at the connecting point 700 ₁ regardless of what circuit is connected between the connecting point 700 ₁ and the DC power supply connection terminal 600. Therefore, at the first frequency f₁, the impedance of the bias circuit 100 as viewed from the bias supply terminal 800 toward the bias circuit 100 is determined by the parallel capacitor 3 and the first parallel inductor 2 ₁.

At the first frequency f₁, a combined admittance Y_(p1) of the parallel capacitor 3 and the first parallel inductor 2 ₁ is expressed by the following formula (2). In this formula, ω_(x) denotes an angular frequency of an x-th frequency f_(x) (ω_(x)=2πf_(x)).

$\begin{matrix} {Y_{p\; 1} = \left( {{{j\omega}_{1}C_{p}} + \frac{1}{{j\omega}_{1}L_{p\; 1}}} \right)} & (2) \end{matrix}$

According to the following formula (3), the inductance L_(p1) of the first parallel inductor 2 ₁ associated with the capacitance C_(p) of the parallel capacitor 3 is determined so that the combined admittance Y_(p1) is zero.

$\begin{matrix} {L_{p\; 1} = \frac{1}{\left( {2\pi\; f_{1}} \right)^{2}C_{p}}} & (3) \end{matrix}$

Since the combined admittance Y_(p1) of the parallel capacitor 3 and the first parallel inductor 2 ₁ is 0, or in other words, a combined impedance Z_(p1) of the parallel capacitor 3 and the first parallel inductor 2 ₁ is infinite, the impedance of the bias circuit 100 as viewed from the bias supply terminal 800 toward the bias circuit 100 is infinite, so that the bias circuit 100 has no effect on the signal at the first frequency f₁.

Next, at a second frequency f₂ different from the first frequency f₁, the short circuit state does not occur at the connecting point 700 ₁, and the inductance L_(p2) of the second parallel inductor is set so that a combined impedance of the parallel capacitor 3, the first parallel inductor 2 ₁, the first series resonator 9 ₁ and the second parallel inductor 2 ₂ is infinite.

Here the inductance L_(s1) and the capacitance C_(s1) are set under the condition of the formula (1) so that the impedance of the first series resonator 9 ₁ at the second frequency f₂ is sufficiently large, or in other words, the admittance of the first series resonator 9 ₁ is substantially zero from the standpoint of the AC signal at the second frequency f₂. Thus, a combined admittance Y_(p2) of the bias circuit 100 (in this case, a combined admittance of the parallel capacitor 3, the first parallel inductor 2 ₁, the first series resonator 9 ₁ and the second parallel inductor 2 ₂) at the second frequency f₂ can be expressed by the following approximate formula (4).

$\begin{matrix} {Y_{p\; 2} \approx \left( {{{j\omega}_{2}C_{p}} + \frac{1}{{j\omega}_{2}\left( {L_{p\; 1} + L_{p\; 2}} \right)}} \right)} & (4) \end{matrix}$

Accordingly, the inductance L_(p2) of the second parallel inductor 2 ₂ is determined according to the following formula (5) so that the combined admittance Y_(p2) is zero.

$\begin{matrix} {L_{p\; 2} = {\frac{1}{\left( {2\pi\; f_{2}} \right)^{2}C_{p}} - L_{p\; 1}}} & (5) \end{matrix}$

Since a combined impedance Z_(p2) of the parallel capacitor 3, the first parallel inductor 2 ₁ and the second parallel inductor 2 ₂ at the second frequency f₂ is infinite, the impedance of the bias circuit 100 as viewed from the bias supply terminal 800 toward the bias circuit 100 is infinite, and thus, the bias circuit 100 have no effect on the signal at the second frequency f₂. Therefore, the DC voltage and/or the direct current from the DC power supply 6 are simultaneously supplied to the active device 180 without affecting the transmission characteristics of the AC signals at the desired two frequencies f₁ and f₂.

As can be seen from the formulas (3) and (5), in design, the frequencies are preferably related according to f₁>f₂.

An impedance Z_(s12) of the first series resonator 9 ₁ at the second frequency f₂ is expressed by the following formula (6).

$\begin{matrix} {Z_{s\; 12} = {\left( {{{j\omega}_{2}L_{s\; 1}} + \frac{1}{{j\omega}_{2}C_{s\; 1}}} \right) = \frac{1 - \frac{\omega_{2}^{2}}{\omega_{1}^{2}}}{{j\omega}_{2}C_{s\; 1}}}} & (6) \end{matrix}$

As can be seen from the formula (6), the impedance Z_(s12) increases as the capacitance C_(s1) of the first resonant capacitor 7 ₁ decreases. On the other hand, as can be seen from the formula (1), if the capacitance C_(s1) decreases, the inductance L_(s1) of the first resonant inductor 8 ₁ has to be increased. Typically, the inductor is formed by a winding. Therefore, if the inductance is increased while maintaining the allowable current of the wire, the size of the inductor increases, and the parasitic capacitance also increases accordingly, so that the desired inductance may not be able to be achieved in a high frequency band. Thus, it is difficult to design a useful inductor that combines a high inductance and a high allowable current. The bias circuit according to the present invention is also expected to handle a high direct current when the bias circuit is applied to a power amplifier. However, since the impedance of the first series resonator 9 ₁ to a DC is infinite as shown by the formula (6), no direct current flows into the first resonant inductor 8 ₁. Therefore, an inductor having a low allowable current can be used as a resonant inductor in the first series resonator 9 ₁. That is, for the bias circuit according to the present invention, it is not difficult to design the capacitor in the series resonator to have a low capacitance and the inductor in the series resonator to have a high inductance, and as a result, the impedance at frequencies other than the resonance frequency of the series resonator can be designed to be high. Thus, the bias circuit can be easily designed.

In addition, as can be seen from the formulas (3) and (5), the inductance L_(p1) of the first parallel inductor 2 ₁ and the inductance L_(p2) of the second parallel inductor 2 ₂ can be decreased by appropriately setting the capacitance C_(p) of the parallel capacitor 3. More specifically, for the bias circuit according to the present invention, the inductance of each parallel inductor can be easily decreased by increasing the capacitance C_(p) of the parallel capacitor 3. Thus, the bias circuit can be easily designed and reduced in size.

FIG. 3 shows a circuit including the bias circuit 100 designed on the condition that f₁=2.5 GHz, and f₂=2 GHz (C_(p)=0.9 pF, L_(p1)=3.9 nH, L_(p2)=1.5 nH, C=1000 pF, C_(s1)=0.6 pF, L_(s1)=6.7 nH). FIG. 4 shows pass characteristics (S21) of an AC signal from the port A to the port B in the bias circuit shown in FIG. 3. As can be seen from the drawing, the loss is extremely low at each frequency, and the bias circuit 100 has substantially no effect on the AC signal.

The bias circuit 100 described above can supply a DC voltage and/or a direct current to an active device simultaneously for two different frequency bands. Next, there will be described a bias circuit 200 according to another embodiment that can supply a DC voltage and/or a direct current to an active device simultaneously for N different frequency bands f₁, f₂, . . . , f_(N) (N denotes a predetermined integer equal to or greater than 2) (see FIG. 5). That is, the bias circuit 100 is the bias circuit 200 in the case of N=2.

The bias circuit 200 includes: a parallel capacitor 3 having a capacitance C_(p) and grounded at one end thereof and connected at the other end thereof to a bias supply terminal 800; and a parallel circuit 3L connected in parallel with the parallel capacitor 3 and connected at one end thereof to the bias supply terminal 800. The parallel circuit 3L includes: a first parallel inductor 2 ₁ having an inductance L_(p(1)) connected at one end thereof to the bias supply terminal 800; a q-th parallel inductor 2 _(q) having an inductance L_(p(q)) and connected at one end thereof to the other end of a (q−1)-th parallel inductor 2 _(q−1) and connected at the other end thereof to one end of a (q+1)-th parallel inductor 2 _(q+1) where q denotes each integer equal to or greater than 2 and equal to or smaller than N−1; an N-th parallel inductor 2 _(N) having an inductance L_(p(N)) and connected at one end thereof to the other end of an (N−1)-th parallel inductor 2 _(N−1) and connected at the other end thereof to a DC power supply connection terminal 600; an m-th series resonator 9 _(m) connected at one end thereof to a connecting point 700 _(m) between an m-th parallel inductor 2 _(m) and an (m+1)-th parallel inductor 2 _(m+1) where m denotes each integer equal to or greater than 1 and equal to or smaller than N−1; a grounding capacitor 4 having a capacitance C and connected at one end thereof to the DC power supply connection terminal 600 and grounded at the other end thereof; and a DC circuit part 10 connected to the DC power supply connection terminal 600. The m-th series resonator 9 _(m) includes: an m-th resonant capacitor 7 _(m) having a capacitance C_(sm) and connected at one end thereof to the connecting point 700 _(m); and an m-th resonant inductor 8 _(m) having an inductance L_(sm) and connected at one end thereof to the other end of the m-th resonant capacitor 7 _(m) and grounded at the other end thereof. The DC circuit part 10 includes: a DC circuit 5, such as a choke coil, connected at one end thereof to the DC power supply connection terminal 600; and a DC power supply 6 that is connected to the other end of the DC circuit 5 and generates a certain DC voltage with respect to the ground potential. The energy of the DC power supply 6 is supplied to an active device 180 via the bias supply terminal 800.

It is supposed that n denotes each integer equal to or greater than 2 and equal to or smaller than N, and a resonance frequency of an (n−1)-th series resonator 9 _(n−1) is referred to as an (n−1)-th frequency f_(n−1).

The frequency f_(n−1), the inductance L_(s(n−1)), and the capacitance C_(s(n−1)) are related to each other according to the following formula (7).

$\begin{matrix} {f_{n - 1} = \frac{1}{2\pi\sqrt{L_{s{({n - 1})}}C_{s{({n - 1})}}}}} & (7) \end{matrix}$

[1] In the case where n is 2, the inductances of the first parallel inductor 2 ₁ and the second parallel inductor 2 ₂ can be determined in the same way as for the bias circuit 100 described above.

[2] In the case where n is an integer equal to or greater than 3 or equal to or smaller than N, although a short circuit state does not occur at the connecting points 700 ₁ to 700 _(n−2), the impedance of the (n−1)-th series resonator 9 _(n−1) as viewed from the connecting point 700 _(n−1) toward the (n−1)-th series resonator 9 _(n−1) is zero (a short circuit state) at the (n−1)-th frequency f_(n−1). Thus, when the bias circuit 200 is viewed from the bias supply terminal 800, the short circuit state always occurs at the connecting point 700 _(n−1) regardless of what circuit is connected between the connecting point 700 _(n−1) and the DC power supply connection terminal 600. Therefore, at the (n−1)-th frequency f_(n−1), the inductance L_(p(n−1)) of the (n−1)-th parallel inductor 2 _(n−1) is determined so that the impedance of the bias circuit 200 as viewed from the bias supply terminal 800 toward the bias circuit 200 (a combined impedance of the parallel capacitor 3, the first to the (n−1)-th parallel inductors 2 ₁ to 2 _(n−1), and the first to the (n−2)-th series resonators 9 ₁ to 9 _(n−2)) is infinite. When the impedance of each of the first to the (n−2)-th series resonators 9 ₁ to 9 _(n−2) at the (n−1)-th frequency f_(n−1) is sufficiently large, or in other words, when the admittance of each of the first to the (n−2)-th series resonators 9 ₁ to 9 _(n−2) is substantially zero from the standpoint of the AC signal at the (n−1)-th frequency f_(n−1), a combined admittance Y_(p(n−1)) of the bias circuit (in this case, a combined admittance of the parallel capacitor 3, the first to the (n−1)-th parallel inductors 2 ₁ to 2 _(n−1), and the first to the (n−2)-th series resonators 9 ₁ to 9 _(n−2)) at the (n−1)-th frequency f_(n−1) is expressed by the following approximate formula (8).

$\begin{matrix} {Y_{p{({n - 1})}} \approx \left( {{{j\omega}_{({n - 1})}C_{p}} + \frac{1}{{j\omega}_{({n - 1})}\left( {L_{p\; 1} + \ldots + L_{p{({n - 1})}}} \right)}} \right)} & (8) \end{matrix}$

Accordingly, the inductance L_(p(n−1)) is determined according to the following formula (9) so that the combined admittance Y_(p(N−1)) is zero.

$\begin{matrix} {L_{p{({n - 1})}} = {\frac{1}{\left( {2\pi\; f_{({n - 1})}} \right)^{2}C_{p}} - \left( {L_{p\; 1} + \ldots + L_{p{({n - 2})}}} \right)}} & (9) \end{matrix}$

Since the combined admittance Y_(p(n−1)) of the parallel capacitor 3 and the first to the (n−1)-th parallel inductors 2 ₁ to 2 _(n−1) at the (n−1)-th frequency f_(n−1) is 0, or in other words, a combined impedance Z_(p(n−1)) of the parallel capacitor 3 and the first to the (n−1)-th parallel inductors 2 ₁ to 2 _(n−1) is infinite, the impedance of the bias circuit 200 as viewed from the bias supply terminal 800 toward the bias circuit 200 is infinite, and thus, the bias circuit 200 have no effect on the signal at the (n−1)-th frequency f_(n−1).

At the N-th frequency f_(N), a short circuit state does not occur at the connecting points 700 ₁ to 700 _(N−1), and the inductance L_(pN) of the N-th parallel inductor 2 _(N) is determined so that the combined impedance of the parallel capacitor 3, the first to N-th parallel inductors 2 ₁ to 2 _(N) and the first to the (N-1)-th series resonators 9 ₁ to 9 _(N−1) is infinite at the N-th frequency f_(N).

When the impedance of each of the first to the (N−1)-th series resonators 9 ₁ to 9 _(N−1) at the N-th frequency f_(N) is sufficiently large, or in other words, when the admittance of each of the first to the (N−1)-th series resonators 9 ₁ to 9 _(N−1) is substantially zero from the standpoint of the AC signal at the N-th frequency f_(N), a combined admittance Y_(pN) of the bias circuit (in this case, a combined admittance of the parallel capacitor 3, the first to N-th parallel inductors 2 ₁ to 2 _(N) and the first to the (N−1)-th series resonators 9 ₁ to 9 _(N−1)) at the N-th frequency f_(N) is expressed by the following approximate formula (10).

$\begin{matrix} {Y_{pN} \approx \left( {{{j\omega}_{N}C_{p}} + \frac{1}{{j\omega}_{N}\left( {L_{p\; 1} + \ldots + L_{pN}} \right)}} \right)} & (10) \end{matrix}$

Accordingly, the inductance L_(pN) of the N-th parallel inductor 2 _(N) is determined according to the following formula (11) so that the combined admittance Y_(pN) is zero.

$\begin{matrix} {L_{pN} = {\frac{1}{\left( {2\pi\; f_{N}} \right)^{2}C_{p}} - \left( {L_{p\; 1} + \ldots + L_{p{({N - 1})}}} \right)}} & (11) \end{matrix}$

Since the combined admittance Y_(pN) of the parallel capacitor 3 and the first to the N-th parallel inductors 2 ₁ to 2 _(N) at the N-th frequency f_(N) is 0, or in other words, a combined impedance Z_(pN) of the parallel capacitor 3 and the first to the N-th parallel inductors 2 ₁ to 2 _(N) is infinite, the impedance of the bias circuit 200 as viewed from the bias supply terminal 800 toward the bias circuit 200 is infinite, and thus, the bias circuit 200 have no effect on the signal at the N-th frequency f_(N).

As can be seen from the formulas (9) and (11), the bias circuit 200 is preferably designed so that f₁>f₂> . . . >f_(N−1)>f_(N). In this case, when the capacitance of each resonant capacitor and the inductance of each resonant inductor are determined so that the impedance of each of the first to the (n−2)-th series resonators 9 ₁ to 9 _(n−2) is sufficiently large (the admittance of each of the series resonators 9 ₁ to 9 _(n−2) is substantially zero) at the (n−1)-th frequency f_(n−1), the impedance of each of the first to the (n−2)-th series resonators 9 ₁ to 9 _(n−2) is also sufficiently large (the admittance of each of the series resonators 9 ₁ to 9 _(n−2) is substantially zero) at the n-th frequency f_(n). This is because the impedance of a resonant capacitor is inversely proportional to the frequency. Therefore, at the n-th frequency f_(n), the capacitance of the resonant capacitor of the (n−1)-th series resonator 9 _(n−1) and the inductance of the resonant inductor of the (n−1)-th series resonator 9 _(n−1) can be determined so that the impedance of the (n−1)-th series resonator 9 _(n−1) is sufficiently large (the admittance of the (n-1)-th series resonator 9 _(n−1) is substantially zero).

An impedance Z_(sny) of an n-th series resonator 9 _(n) at the y-th frequency f_(y) (n<y≦N) is expressed by the following formula (12).

$\begin{matrix} {Z_{sny} = {\left( {{{j\omega}_{y}L_{sn}} + \frac{1}{{j\omega}_{y}C_{sn}}} \right) = \frac{1 - \frac{\omega_{y}^{2}}{\omega_{n}^{2}}}{{j\omega}_{y}C_{sn}}}} & (12) \end{matrix}$

As can be seen from the formula (12), the impedance Z_(sny) increases as a capacitance C_(sn) of an n-th resonant capacitor 7 _(n) decreases. On the other hand, as can be seen from the formula (7), if the capacitance C_(sn) decreases, an inductance L_(sn) of an n-th resonant inductor 8 _(n) has to be increased. However, since the impedance of the n-th series resonator 9 _(n) to a direct current is infinite as described above, no direct current flows into the n-th resonant inductor 8 _(n). Therefore, an inductor having a low allowable current can be used as a resonant inductor in the n-th series resonator 9 _(n). That is, in designing the bias circuit capable of simultaneous biasing for N different frequencies according to the present invention, it is not difficult to design the capacitors in the series resonators to have low capacitances and the inductors in the series resonators to have high inductances, and as a result, the impedance at frequencies other than the resonance frequencies of the series resonators can be designed to be high. Thus, the bias circuit can be easily designed.

In addition, as can be seen from the formulas (3), (5), (9) and (11), the inductance of each of the first to the N-th parallel inductors 2 ₁ to 2 _(N) can be decreased by appropriately setting the capacitance C_(p) of the parallel capacitor 3. More specifically, for the bias circuit according to the present invention, the inductance of each parallel inductor can be easily decreased by increasing the capacitance C_(p) of the parallel capacitor 3. Thus, the bias circuit can be easily designed and reduced in size.

FIG. 6 shows a circuit including a bias circuit 200 w, which is the bias circuit 200 in the case of N=4 that is designed to have operating frequencies f₁=2.5 GHz, f₂=2 GHz, f3=1.5 GHz and f₄=1 GHz (C_(p)=0.9 pF, L_(p1)=3.7 nH, L_(p2)=1.6 nH, L_(p3)=1.6 nH, L_(p2)=2.9 nH, C=1000 pF, C_(s1)=0.6 pF, L_(s1)=6.7 nH, C_(s2)=1.2 pF, L_(s2)=5.3 nH, C_(s3)=2.5 pF, L_(s3)=4.4 nH). FIG. 7 shows pass characteristics of an AC signal from a port A to a port B in the bias circuit shown in FIG. 6. As can be seen from the drawing, the loss is extremely low at each frequency, and the bias circuit 200 w has substantially no effect on the AC signal.

Although the bias circuit according to the present invention can supply a DC voltage and/or a direct current to an active device simultaneously for AC signals in N different frequency bands, the AC signals in the N frequency bands do not always have to flow between the port A and the port B. The bias circuit according to the present invention can supply a DC voltage and/or a direct current even when only AC signals in some of the N frequency bands flow between the port A and the port B.

In the configurations according to the embodiments described above, the m-th series resonators 9 _(m) are shown as including: the m-th resonant capacitor 7 _(m) having a capacitance C_(sm) and connected at one end thereof to the connecting point 700 _(m); and the m-th inductor 8 _(m) having an inductance L_(sm) and connected at one end thereof to the other end of the m-th resonant capacitor 7 _(m) and grounded at the other end thereof. However, the present invention is not limited to such configurations. For example, all or some of the N−1 series resonators in the bias circuit 200 may be configured in the same way as a second series resonator 91 ₂ shown in FIG. 8 that includes: an m-th resonant inductor 8 _(m) having an inductance L_(sm) and connected at one end thereof to the connecting point 700 _(m); and an m-th resonant capacitor 7 _(m) having a capacitance C_(sm) and connected at one end thereof to the other end of the m-th resonant inductor 8 _(m) and grounded at the other end thereof.

The foregoing description of the embodiments of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled. 

What is claimed is:
 1. A bias circuit, comprising: a bias supply terminal configured to supply a direct-current voltage and/or a direct current to an active device; a parallel capacitor connected at one end thereof to the bias supply terminal and grounded at the other end thereof; and a parallel circuit connected in parallel with the parallel capacitor and connected at one end thereof to the bias supply terminal, wherein letting N denote a predetermined integer equal to or greater than 2, and m denote each integer equal to or greater than 1 and equal to or smaller than N−1, the parallel circuit comprises: a direct-current power supply connection terminal connected to a direct-current circuit part including a direct-current power supply; N series inductors connected in series with each other between the bias supply terminal and the direct-current power supply connection terminal; and N−1 series resonators, an m-th one of the N−1 series resonators from the bias supply terminal side includes an m-th resonant capacitor and an m-th resonant inductor and is connected at one end thereof to a connecting point between an m-th and (m+1)-th ones of the N series inductors from the bias supply terminal side and grounded at the other end thereof, wherein letting n denote an integer equal to or greater than 2 and equal to or smaller than N+1 and letting a resonance frequency of the (n−1)-th series resonator be referred to as an (n−1)-th frequency f_(n−)1, for n=2, an inductance of a first one of the N series inductors and a capacitance of the parallel capacitor are set so that a combined admittance of the parallel capacitor and the first one of the N series inductors is zero at a first frequency f₁; for n equal to or greater than 3 and equal to or smaller than N+1, an inductance of each of the resonant inductors and a capacitance of each of the resonant capacitors are set so that a combined admittance of the parallel capacitor, the first to the (n−1)-th series inductors and the first to an (n−2)-th series resonators is zero at the (n−1)-th frequency f_(n−1).
 2. The bias circuit according to Claim 1, wherein the inductance of each of the resonant inductors and the capacitance of each of the resonant capacitors are set so that an impedance of each of the first to the m-th series resonators is sufficiently large at an (m+1)-th frequency f_(m+1).
 3. The bias circuit according to Claim 1 or 2, wherein f₁ >f₂ >. . . >f_(N−1) >f_(N). 